The operation of any scanning-head device in traversing or moving the printhead across the medium to discharge ink droplets does present some obstacles to precise positioning of the printed marks, and also to best image quality. In order to describe these obstacles it will be helpful first to set forth some of the context in which these systems operate.
In many printing devices, position information is derived by automatic reading of graduations along a scale or so-called "encoder strip" (or sometimes "code strip") that is extended across the medium. The graduations typically are in the form of opaque lines marked on a transparent plastic or glass strip, or in the form of solid opaque bars separated by apertures formed through a metal strip.
Such graduations typically are sensed electro-optically to generate an electrical waveform that may be characterized as a square wave, or more rigorously a trapezoidal wave. Electronic circuitry responds to each pulse in the wave train, signaling the pen-drive (or other marking-head-drive) mechanism at each pixel location-that is, each point where ink can be discharged to form a properly located picture element as part of the desired image.
These data are compared, or combined, with information about the desired image triggering the pen or other marking head to produce a mark on the printing medium at each pixel location where a mark is desired. As will be understood, these operations are readily carried out for each of several different ink colors, for printing machines that are capable of printing in different colors.
In addition to this use of the encoder derived signal as an absolute physical reference for firing the pens, the frequency of the wave train is ordinarily used to control the velocity of the pen carriage. Some systems also make other uses of the encoder signal such as, for example, controlling carriage reversal, acceleration, mark quality, etc. in the end zones of the carriage travel, beyond the extent of the markable image region.
Now, standardized circuitry for responding to each pulse in the encoder derived signal is most straightforwardly designed to recognize a common feature of each pulse. Thus some circuits may operate from a leading (rising) edge of a pulse, others from a trailing (falling) edge-but generally each circuit will respond only to one or the other, not both.
Such circuits have been developed to a highly refined stage. Accordingly it is cost-effective and otherwise desirable to employ one of these well refined, already existing circuits relative to such compensation; however, in adapting such a preexisting design, several problems arise relative to encoder dimensional tolerances and ink droplet time of flight from nozzle to print medium. Each of these problems will now be examined in greater detail.
(a) Encoder dimensional tolerances
As noted earlier, the position of the pen carriage unit is monitored utilizing a detector that reads a position encoder. In this regard, encoder-reading circuitry processes the encoder reader data to produce pulses each time the carriage unit moves a fixed distance, usually on the order of 1/75th or 1/150th of an inch. However, the drop placement on the print medium must be placed on spatial boundaries that are much smaller, such as down to 1/4800th of an inch or even smaller. Unfortunately, the position encoder is not nearly this precise. Moreover, other problems are associated with the encoder dimensional tolerances. For example, the encoder-reading circuitry is triggered from the falling edges of the initial encoder-derived wave train. The alternating opaque markings and transparent segments (or solid bars and orifices) of the encoder strip are arranged in time alignment with the signals that result from reading of those features by a transmissive optical emitter/detector pair.
It will be understood that, in selecting the point at which a mark should be made, it is possible to make allowance for the nominal width of the transparent segment. For example, the firing of a pen could be delayed by a period of time automatically calculated from the nominal width of the transparent segment divided by the carriage velocity. Although both these pieces of information are available during operation of the system, the results of this method would be unsatisfactory because of preferred manufacturing procedures for creation of the encoder strip. These procedures arise from economics related to dimensional requirements, as follows.
Thus, in making an encoder strip, the dimension, which is most important to hold to highest precision, is the overall periodicity of the alternating opaque bars and transparent segments, i.e., the periodicity dimension that gives rise to a full wavelength of the wave train. The two internal dimensions of each mark-and-transparent-segment pair, namely, the length of the bar and the length of the transparent segment, are much less important.
In a unidirectional printing machine, only the distance between falling edges (or alternately rising edges) has any importance, provided only that (1) the distance from each falling edge to its next associated rising edge is great enough to permit the sensing apparatus to recognize the falling edge; and (2) the distance from each rising edge to its next associated falling edge is great enough to permit the sensing apparatus to reset itself in preparation for sensing the falling edge.
More specifically, the dimensional accuracy of the encoder-strip features are plus-or-minus only one percent for the full periodic pattern width, but plus-or-minus ten to twenty percent for the opaque bar width alone. While it would be entirely possible to manufacture an encoder strip with much finer precision in the internal dimensions just mentioned, an encoder strip so made would be substantially more expensive.
(b) Time-of-Flight
A certain amount of time elapses between the issuance of a mark-command pulse to a print head and the mark actually being created on the printing medium. For instance, in an inkjet printer, some time elapses between: the issuance of a fire-command pulse, approximately at an encoder-wave train falling edge, to a pen nozzle and the instant when a resulting ink drop actually reaches the medium.
During this time, however, the carriage and pen continue to move across the printing medium, and, in the case of an inkjet device, so does the ink drop, even after leaving the pen. The initial velocity component of the drop along the scanning axis or dimension, when scanning forward is very closely equal to the carriage velocity. This velocity likely decreases while the drop travels in the orthogonal axis or dimension toward the printing medium, but nevertheless, some forward movement or displacement of the ink drop along the scanning axis does occur before the drop reaches the medium to form an ink spot.
(c) Pen Datum in Carriage
(d) Drop Trajectory and Velocity
(e) Tail Distortion
Another problem associated with time-of-flight effects is dot tail distortion created when a high-speed ink droplet impacts a stationary print medium. In this regard, at lower carriage velocity speeds there is no or substantially little visible dot distortion due to time-of-flight impact. However, as will be explained hereinafter in greater detail, as the velocity of the carriage unit is increased to achieve a higher throughput, substantial dot distortion can result at the higher carriage velocity speeds.
Therefore it would be highly desirable to have a new and improved system that corrects for variations in the ink drop flight path due to different carriage velocity, and that also adjusts the dot formation due to the increased carriage velocity rates. Such a new an improved system should also accurately determine ink droplet positioning to ensure that such droplets hit the medium at a proper position.